1. Field of the Invention
The present invention relates to a method of removing common noise and a method of detecting touch information.
2. Discussion of Related Art
Currently, resistive touch screens, surface acoustic wave (SAW) touch screens, and capacitive touch screens are mainly in use. Capacitive touch screens are capable of sensing multiple touches and have excellent durability, sensibility, and so on. Therefore, there is a current trend toward adopting capacitive touch screens as the major input units of mobile devices.
A capacitive touch screen senses a change in the amount of charge caused by a user's interference in capacitive sensors on a touch screen panel, thereby recognizing the user's input. Capacitive touch screens are classified into a self-capacitive type and a mutual-capacitive type according to charge accumulation methods. In a self-capacitive touch screen, each capacitive sensor constitutes one electrical conductor and forms electrified surfaces together with a reference ground surface outside a touch screen panel. On the other hand, in a mutual-capacitive touch screen, two electrical conductors on a touch screen panel form electrified surfaces and function as one capacitive sensor.
A general self-capacitive touch screen employs an orthogonal X/Y disposition of electrical conductors, and in this case, each capacitive sensor functions as a line sensor. Therefore, every time the touch screen attempts to sense a touch, only one piece of X-sensing information and one piece of Y-sensing information are provided by an X-line sensor group and a Y-line sensor group, respectively. For this reason, the general self-capacitive touch screen is capable of sensing and tracking a single touch but incapable of supporting multiple touches. A mutual-capacitive touch screen also employs an orthogonal X/Y disposition of electrical conductors, but is different from the self-capacitive touch screen in that each capacitive sensor is configured in the form of a grid sensor at every position where electrical conductors cross at right angles and responses of all grid sensors are separately sensed upon attempting to detect a user's input on the touch screen. Since the respective grid sensors correspond to different X/Y coordinates and provide separate responses, in the mutual-capacitive touch screen, a user's input information may be extracted from an X/Y-sensing information set provided by an X/Y grid sensor set, and the user's multiple touches may be sensed and tracked.
A general mutual-capacitive touch screen panel has the following electrical conductor configuration and sensing method. First electrodes consisting of electrical conductors extending in any one direction and second electrodes consisting of electrical conductors extending in a direction crossing the first electrodes at right angles form mutual-capacitive sensors with a dielectric material interposed between the first electrodes and the second electrodes. When a distance between a first electrode and a second electrode is d, an area of electrified surfaces is a, and the equivalent permittivity of all dielectric materials between the electrified surfaces is ∈, a capacitance C of each sensor is defined as C=∈*a/d, and an amount Q of charge accumulated in the sensor and a potential difference (voltage) V applied to two electrodes/electrified surfaces has a relationship of Q=CV. When a user approaches a sensor, interference occurs in an electric field formed between two electrodes and disturbs the accumulation of charge in the sensor. Then, the amount of charge accumulated in the sensor decreases, and as a result, capacitance is reduced. This may be understood as a change in capacitance resulting from a change in the equivalent permittivity between the electrified surfaces caused by the user's approach, but is actually a physical phenomenon in which a part of the electric field between the electrified surfaces is shunted due to the user's approach and the accumulated charge is reduced. When an alternating current (AC) waveform is applied to one electrified surface of a sensor by connecting an AC voltage source to the first electrode, a change ΔQ in the amount of charge corresponding to ΔQ=CΔV occurs with respect to C that is changed according to the degree of a user's approach, and a read-out circuit connected to the second electrode converts the change ΔQ into current or voltage. Information converted in this way is generally subjected to signal processing operations, such as noise filtering, demodulation, digitizing, and accumulation, and then used in a coordinate tracking algorithm and a gesture recognition algorithm. U.S. Pat. No. 7,920,129 discloses such a capacitive touch sensitive panel.